1. Technical Field
The present disclosure relates to an apparatus for controlling an inverter.
2. Description of the Related Art
A medium voltage inverter refers to an inverter using an input power greater than 600 VRMS of line voltage, and its rated power capacity ranges from several hundreds of kW to several tens of MW. Medium voltage inverters are commonly used in fans, pumps, compressors, etc. Among such medium voltage inverters, a cascaded multi-level inverter is frequently used, of which output phase-voltages have three or more output voltage levels. The magnitude and the number of output voltage levels of a multi-level inverter are determined depending on the number of power cells thereof. Each of the power cells uses an isolated input voltage.
Typically, a medium voltage electric motor driven by a medium voltage inverter has very high inertia. Accordingly, when an error takes place in an input power or service interruption occurs, it takes long time for the electric motor to completely stop operating for restarting. In order to reduce the time period for restarting, a voltage may be applied according to a voltage/frequency ratio (V/f) during the rotation of the electric motor. However, this may result in a large inrush current to cause malfunction in an inverter or an electric motor.
For this reason, in order to reduce time period for restarting and avoid malfunction in an inverter or an electric motor, a voltage measuring device is employed. However, when the voltage measuring device consisting of a passive element such as a resistor is employed for saving cost, there is a problem in that an error in the measured voltage takes place due to an error in resistance. In addition, an inverter has factors causing voltage drop such as pulse width modulation (PWM) of an output voltage and dead time. In particular, for a medium voltage inverter having a low switching frequency, it is difficult to learn an output voltage precisely.
For these reasons, there is a problem in restarting when the measured voltage is used as the output voltage from the inverter.